After installation there's xcss command line utility, which generates a CSS
from a given xCSS module:

% xcss ./index.css > bundle.css

Run xcss --help to see more options:

Usage: xcss [options] <filename>
Compile xCSS to CSS and print the result.
Options:
-c, --compress Compress the resulted stylesheet
--object-model Print the result of translation from xCSS to JavaScript
-h, --help Show this message and exit